Diesel engines are powerplants used on many trucks that are presently being manufactured in North America.
Typically, diesel engines are equipped with single or two-stage turbochargers. A two-stage turbocharger comprises high- and low-pressure turbines in series flow relationship in the exhaust system that operate high- and low-pressure compressors in series flow relationship in the intake system to develop boost is one example of a turbocharger. A single-stage stage turbocharger has only a single turbine and a single compressor.
Diesel engine combustion must be precisely controlled to minimize emissions. Among the factors in determining emission levels is the presence of excess oxygen in the combustion process. Exhaust gas recirculation (EGR) is the process of re-circulating a portion of an engine's exhaust back into the engine's cylinders, and it has been used to reduce peak combustion temperatures, lower excess oxygen levels, and reduce NOx emissions.
Increasingly stringent emission regulations require increasing EGR flow rates to avoid the use of expensive after treatment technologies.
U.S. published patent application 2008/0078176 and U.S. Pat. No. 6,973,786 describe engine control systems for turbocharged diesel engines. These documents describe strategies for control of turbochargers and EGR valves.
There are different known methods to “drive” EGR, that is, to ensure that sufficient EGR flow rate is maintained with respect to the fresh air flow rate into the intake manifold. Because EGR gas flow must be driven through the EGR cooler and conduits, pressure drops can reduce the maximum flow rate of EGR gas given the pressure available in the exhaust manifold. Some methods used alone or in combination are:
a. Use of the air intake throttle valve; restricting intake air using the throttle can be used to drive EGR; however, at the expense of higher fuel consumption.
b. Use of cold side EGR valve (i.e., pneumatic, hydraulic, electrical) or a check valve, such as a Reed valve. These types of valves used at the EGR gas outlet port let the EGR gases flow into the EGR/air mixer and prevent the charged air boost pressure (when higher than the EGR pressure) from reversing the EGR flow.
c. Use of pulsating exhaust manifolds. Exhaust gases pressure varies with a sinusoidal form (pulses with peaks and outlet). When the exhaust gas pressure is at the peak of the pulse (highest value) it is used to overcome the charge air boost pressure, thus creating the mix of fresh charged air with EGR gases. In order to be able to use the pulses of each cylinder, exhaust manifolds are designed in a manner that allow the connection of exhaust runners with pressure waves that are in phase. This type of design commonly called “split manifolds” are used in combination with split EGR hot side tubes (from exhaust manifolds to EGR), split bundles in the EGR cooler core and split EGR cold tubes (from EGR to the EGR/air mixer).
The present inventors recognize that it would be desirable to provide an EGR driving system that is economical to manufacture, and effective and reliable in operation.